T cell differentiation in the thymus is dependent on signaling by both T cell antigen receptors and by cytokine receptors. Our current understanding of how lineage choice is determined in the thymus is built around the concept that signaling by the T cell antigen receptor prevents signaling by cytokine receptors, and that cytokine signaling only occurs after signaling by the T cell antigen receptor has ceased. Despite its central role in T cell development, it is not known how T cell antigen receptor signaling 'desensitizes'the cytokine receptor so that it is no longer able to transmit signals into the cell. Consequently, we have undertaken a systematic examination of the molecular basis by which cytokine receptor signaling is impaired by TCR signals in developing T cells in the thymus and in mature T cells in the periphery.